I'm on an old Lenovo T60 Think Pad Windows XP with no virus protection and probably a 1,000 or more cookies still in the oven, no problems posting (this will be 3rd one today).Can't help but think your problem is an intermitent glich with your ISP.
It seems that I am/was not the only one having these problems, so I doubt that it is my ISP. I have had ISP issues in the past, but that's usually when they hose up the DNS. A reboot of the modem fixes that. The error we were getting appeared to be coming from the "Simple Machines" forum software.
I would try to post something several times. Each time it would refuse. Then I would come back later and it would work flawlessly. Then, figuring everything is fixed, I would move to another section and the error would come back.
But, it might have cleared up now because, I haven't had any problems for a while.
Now it all seems to be working consistently.